This invention generally relates to the field of data communication networks. More particularly, this invention pertains to wireless local area networks (LANs) for a data communication network having a number of users exchanging data between individual remote stations and a central station over a single optical infrared channel.
A multipoint digital communication network typically consists of a number of remote stations which communicate with a central station over one or more two-way communication channels. For example, personal computers are typically connected to a wide variety of peripherals or other computers via wire cables, i.e., a hard-wired communication link. Moreover, local area networks (LANs) are often used to integrate remote terminals that are located at the same site. Depending upon the number of users, distance between terminals, number of peripherals, frequency of system reconfiguration, portability of the remote stations, etc., the hard-wired cable system may not be practical for a given application. Hence, various wireless communication technologies have been employed, particularly when a system includes a large number of users and/or portable, hand-held computer devices.
Among the more common wireless technologies are narrow-band radio frequency (RF) systems, spread spectrum RF, ultrasonic, and infrared optical. Radio frequency systems are often significantly degraded by electromagnetic noise and interference, as well as by large signal amplitude variations and multipath interference. Moreover, RF systems are typically subject to governmental licensing and regulation. Alternative wireless systems employing ultrasonic sound waves experience severe problems with the complete loss of signals due to nulls in the transmission path.
Optical-infrared communication, however, is not affected by electromagnetic interference, and is much less susceptible to mutipath interference. Furthermore, optical systems are inherently secure (since the infrared light does not penetrate walls), have no known health or safety effects (at low power levels), and are not subject to F.C.C. regulation. Moreover, infrared transceivers draw relatively low currents, which is particularly important with respect to hand-held battery-powered portable computers. Thus, the use of infrared light as the wireless medium is well suited to such applications.
One practical infrared LAN was taught by Lee in U.S. Pat. No 5,099,346, (Lee) assigned to the assignee of the present invention. Under Lee a number of computers and peripherals (remote stations) were interconnected using an infrared optical communication system. While Lee worked well, the infrared devices of Lee were line-of-sight devices. Line-of-sight devices are not particularly well suited to palmtop computers or other data processing devices requiring great mobility. Because of the importance of mobile communication between data processing devices in enclosed areas and in the utility of palmtop computers in general, a need exists for a means of exchanging data between mobile and stationary data processing devices that is more flexible than line-of-sight devices and yet not susceptible to electromagnetic interference.
Accordingly, a primary object of the present invention is to provide a method for using existing infrared transmitting and receiving apparatus, but in an arrangement to enable the transceiving of infrared signals indirectly as well as line-of-sight.
A further and more particular object of this present invention is to provide a radiation filter for a photodiode detector of the transceivers which blocks radiation outside the infrared region thereby increasing sensitivity of the infrared receivers.
A further and more particular object of this present invention is to provide an infrared detector within an infrared receiver whose speed and performance is not dependent upon changes in signalling light levels.
A further and more particular object of this present invention is to provide a means of locating a portable remote station based upon measured parameters of the portable remote station.